diff --git a/newlib/libc/machine/arm/strlen-stub.c b/newlib/libc/machine/arm/strlen-stub.c
index bcd3d2d46..ea45a3789 100644
--- a/newlib/libc/machine/arm/strlen-stub.c
+++ b/newlib/libc/machine/arm/strlen-stub.c
@@ -32,11 +32,14 @@
 #include <limits.h>
 
 #if defined __OPTIMIZE_SIZE__ || defined PREFER_SIZE_OVER_SPEED
-#if defined __thumb__ && !defined __thumb2__
+#if __ARM_ARCH_ISA_THUMB == 2
+/* Implemented in strlen.S.  */
+
+#elif defined (__ARM_ARCH_ISA_THUMB)
 /* Implemented in strlen.S.  */
 
 #else
-/* Implemented in strlen.S.  */
+#include "../../string/strlen.c"
 
 #endif
 
diff --git a/newlib/libc/machine/arm/strlen.S b/newlib/libc/machine/arm/strlen.S
index 57371453a..0435fb2de 100644
--- a/newlib/libc/machine/arm/strlen.S
+++ b/newlib/libc/machine/arm/strlen.S
@@ -27,11 +27,14 @@
 #include "acle-compat.h"
 
 #if defined __OPTIMIZE_SIZE__ || defined PREFER_SIZE_OVER_SPEED
-#if defined __thumb__ && !defined __thumb2__
+#if __ARM_ARCH_ISA_THUMB == 2
+#include "strlen-thumb2-Os.S"
+
+#elif defined (__ARM_ARCH_ISA_THUMB)
 #include "strlen-thumb1-Os.S"
 
 #else
-#include "strlen-thumb2-Os.S"
+  /* Implemented in strlen-stub.c.  */
 
 #endif